THERMODYNAMIC SYSTEMS AND PROCESSES
original conditions if the process was reversed. For example, an automobile engine does not give
back the fuel it took to drive up a hill as it coasts back down the hill.
There are many factors that make a process irreversible. Four of the most common causes of
irreversibility are friction, unrestrained expansion of a fluid, heat transfer through a finite
temperature difference, and mixing of two different substances. These factors are present in real,
irreversible processes and prevent these processes from being reversible.
An adiabatic process is one in which there is no heat transfer into or out of the system. The
system can be considered to be perfectly insulated.
An isentropic process is one in which the entropy of the fluid remains constant. This will be true
if the process the system goes through is reversible and adiabatic. An isentropic process can also
be called a constant entropy process.
When a gas undergoes a reversible process in which there is heat transfer, the process frequently
takes place in such a manner that a plot of the Log P (pressure) vs. Log V (volume) is a straight
line. Or stated in equation form PVn = a constant. This type of process is called a polytropic
process. An example of a polytropic process is the expansion of the combustion gasses in the
cylinder of a water-cooled reciprocating engine.
A throttling process is defined as a process in which there is no change in enthalpy from state
one to state two, h1 = h2; no work is done, W = 0; and the process is adiabatic, Q = 0. To better
understand the theory of the ideal throttling process lets compare what we can observe with the
above theoretical assumptions.
An example of a throttling process is an ideal gas flowing through a valve in midposition. From
experience we can observe that: Pin > Pout, velin < velout (where P = pressure and vel = velocity).
These observations confirm the theory that hin = hout. Remember h = u + Pv (v = specific
volume), so if pressure decreases then specific volume must increase if enthalpy is to remain
constant (assuming u is constant). Because mass flow is constant, the change in specific volume
is observed as an increase in gas velocity, and this is verified by our observations.